Solid lubricant, especially for friction linings, friction lining mixtures

ABSTRACT

A solid lubricant, in particular for friction linings, on the basis of metal sulphides, which contains or consists of at least one compound of the formula 
     
         M1M2.sub.m M3.sub.n S.sub.x 
    
     wherein M1, M2, and M3 each represent one metal of the series of Ti, V, Mn, Fe, Cu, Zn, Mo, W, Sb, Sn and Bi, S denotes sulphur, and the subscripts comprise the ranges of I=1-5, m=1-5, n=0-5 and x=2-8, as well as friction lining mixtures and friction linings containing the same.

The present invention relates to a solid lubricant, in particular forfriction linings, on the basis of metal sulphides, as well as tofriction lining mixtures and friction linings containing the same, suchas brake or clutch linings.

For instance, MoS₂ being a sulphide has long been known as a solidlubricant for the formation of a solid lubricant film between surfacesintended to glide with regard to each other. MoS₂ and other metalsulphides are also used as components of gliding compositions consistinge.g. primarily of PTFE, i.e. for instance plain bearing half liners orslide bushings, in order to reduce surface friction.

Metal sulphides are also used as solid lubricants in a completelydifferent field, namely in the production of friction elements, such asbrake blocks, brake shoes, brake and clutch linings, the purpose ofwhich is not the prevention of friction but the generation of friction.

Thus it is obvious that the aim of using solid lubricants in frictionlinings is not the reduction of friction, but rather the stabilisationof the frictional behaviour in relation to time, resulting in thereduction of abrasive processes and in a positive effect on wear andvibration characteristics. A highly desirable side effect when usingcertain solid lubricants is a considerable stabilisation of thecoefficient of friction, i.e. the detrimental dependence of thecoefficient of friction and thus of braking efficiency on temperature,pressure load and speed, is suppressed to a major extent.

Those best known among the special solid lubricants used in frictionlinings are graphite and molybdenum disulphide, But there are a numberof other metal sulphides used almost exclusively in friction linings,differing from those mentioned above primarily with respect to thestabilisation of the coefficient of friction.

Publications to be mentioned in this context are EP497,751 andAT-399,162, which are concerned with various metal sulphides andcombinations thereof, respectively.

One well-known and frequently used solid lubricant for friction liningsis lead sulphide, which will however be available for futureapplications to a constantly reduced extent because of the increasingecological sensibilization against heavy metals. But according to thepresent art, the good and favourable characteristics of this materialmake it practically impossible to use a substitute for lead sulphidewithout impairing the quality of the friction lining.

For many years there have been attempts to produce synergistic mixtureshaving extraordinary effects by combining various solid lubricants onthe basis of graphites, sulphides, fluorides, phosphates etc. (see DE 2514 575, DE 35 13 031 or EP-328,514). However, these or similarcombinations have so far not been successful in achieving an effect thatis satisfactory in every respect.

Even the combinations of AT-399162 on the basis of the sulphides ofcopper with those of zinc, antimony, molybdenum, tin, tungsten andtitanium, in spite of their generally favourable properties, are notfully satisfactory with respect to their effect either. Even less commonsulphides, like that of bismuth, are no satisfactory solution.

Now it has surprisingly been found that a number of bimetal and trimetalsulphides, when used as solid lubricants, in particular in frictionlinings, give excellent results, among others much more favourableresults than single-metal sulphide mixtures of analogous composition.

Accordingly, the present invention primarily proposes a solid lubricant,in particular for friction linings that are preferably resin-bound, onthe basis of metal sulphides, characterized in that the solid lubricantcontains or consists of at least one compound of the formula

    M1.sub.l M2.sub.m M3.sub.n S.sub.x

wherein M1, M2, and M3 each represent one metal of the series of Ti, V,Mn, Fe, Cu, Zn, Mo, W, Sb, Sn and Bi, S denotes sulphur, and thesubscripts comprise the ranges of I=1-5, m=1-5, n=0-5 and x=2-8.

Preferably the solid lubricant is a polyphase sulphide mixture, inparticular a combination of at least one compound of the formula

    M1.sub.l M2.sub.m M3.sub.n S.sub.x

with one or several sulphides of Ti, V, Mn, Fe, Cu, Zn, Mo, W, Sb, Snand Bi.

Furthermore the invention proposes the corresponding friction liningmixtures and friction linings containing such solid lubricants.

Bimetal sulphides and trimetal sulphides of the above group aredescribed, the literature and exist as minerals as well; for the purposeof exemplification only (giving the corresponding minerals in brackets),Cu₂ FeSnS₄ (stannite), Cu₂ FeSn₃ S₈ (rhodostannite), Cu₃ SnS₄(curmanite), Mn₂ SnS₄, SnFe₂ S₄, Cu₂ TiS₂ may be mentioned.

The bimetal and trimetal sulphides proposed as solid lubricants by theinvention may be produced according to usual processes for theproduction of sulphides, namely sulphidization (heating of metal powderswith sulphur or polysulphides) or reaction of hydroxides or oxides withammonium sulphide or H₂ S according to a wet-chemical or dry method.

Typically the polymetal sulphides may be produced via a melt phase, e.g.by fusing together binary pure sulphides while excluding air, This oftenresults in mixtures of several different phases. The wet-chemicalprecipitation of polymetal sulphides is described the literature aswell. It has been shown that proportional amounts of polymetal sulphidesare also formed in the course of sulphidizing metal alloys and metalpowder mixtures, which can be ascertained by X-ray diffraction or usingan electron microscope by means of a microprobe.

In the following, the invention will be described in more detail by wayof non-limiting examples.

EXAMPLES

A typical disk brake lining recipe was used for comparing the sulphidesaccording to the invention with known solid lubricants. The batch, notyet containing sulphides is mixed in a plough blade mixer with a knifehead. In order to produce the specimens, the solid lubricants to becompared are subsequently blended into the premix in constantproportions, pressed in a pressure- and temperature-controlledlaboratory press so as to form disk brake linings common in vehicles,and tested on a Krauss torque stand.

The test recipe has the following composition:

    ______________________________________    steel wool           10% by weight    metal powder         15%    fibers               9%    organic components   11%    friction materials   9%    fillers              25%    graphite             13%    sulphide             8%    ______________________________________

During the test program particular attention was given to the stabilityof the coefficient of friction and wear characteristics at high loads,as the favourable characteristics of the sulphides are especiallyeffective under these conditions.

The following test program was chosen:

1st) running-in phase with 100 stops for conditioning of the surfaces;

2nd) v-test: 6 cycles with 5 stops each in series, each cycle startingat 100° C., followed by 7 cycles with 10 stops each in series, at atemperature corresponding to 140 km/h and a pressure of 20 bar;

3rd) p-test: by analogy to v-test at a temperature corresponding to 60km/h and a pressure of 50 bar.

During the 10-stop cycle the temperature rises to about 550° C., so thata temperature profile of 100-550° C. is covered in the course of thetest. The change of the coefficient of friction within this temperatureprofile and the total wear of the linings for the test are determinedseparately for v- and p-tests as a characteristic value.

The results of the test series can be seen in the following table. Inthe table, PW-v and PW-p refer to the wear of linings in v- and p-tests,in each case evaluated as reduction of weight in grams per lining, dMy-vand dMY-p refer to the variation of the coefficient of friction withinone test cycle from 100 to 550° C., positive values indicating areduction of the coefficient of friction at high temperatures.

    ______________________________________    Solid lubricant                   dMy-v    dMy-p     pW-v PW-p    ______________________________________    Comparative products:    PbS            -0,12    -0,02    6,9   6,2    Cu.sub.2 S     -0,02    0,12     5,0   9,2    MoS.sub.2      0,11     0,08     13,5  12,5    FeS            0,05     0,05     6,5   8,7    ______________________________________

Example 1

In order to illustrate the differences between mixtures and inventivecombinations, a mixture of Cu₂ S (56%) and ZnS (44%) (mixture 1) iscompared to combination 1. The latter is obtained by sulphidizing brasspowder (Cu/Zn 60:40). Combination 1 and mixture 1 contain the sameproportions of Cu, Zn and S, but are clearly different in their X-raydiffraction patterns.

    ______________________________________    Solid lubricant                dMy-v    dMy-p    PW-v   PW-p    ______________________________________    Mixture 1   0,03     0,18     13,7   15,2    Combination 1                0,02     0,14     12,5   11,8    ______________________________________

There are clear improvements in the lining characteristics at highpressure, resulting in a smaller decrease of the coefficient of frictionand lesser wear as compared to the mixture of the binary sulphides.

Example 2

Here mixtures of binary sulphides are compared to combinations havingthe same proportions of elements, but having been produced bysulphidizing metal powder mixtures, where the ternary and quaternaryphases according to the invention could form. The following compositionswere tested:

    ______________________________________    Composition               Cu.sub.2 S                      TiS.sub.2                               MnS  SnS    FeS  Bi.sub.2 S.sub.3    ______________________________________    Mixture 2  80     20    Mixture 3  45              50   5    Mixture 4  50                   40     10    Mixture 5  50                          20   30    Mixture 6  30                   40          30    ______________________________________

The difference in the molecular composition between mixtures 2-6 andcombinations 2-6 manifests itself in the appearance of ternary andquaternary sulphide phases in the combinations. The following ternaryand quaternary phases may be detected by X-ray diffraction:

Combination 2: CuTi₂,05 S₄

Combination 3: Mn₂ SnS₄

Combination 4: Cu₅ SnS₄, Cu₅ Sn₂ S₇, Cu₂ FeSnS₄, Cu₂ FeSn₃ S₈

Combination 5: CuFe₂ S₃, Cu₅ FeS₄, CuBiS₂

Combination 6: Cu₂ SnS₃, Cu₃ SnS₄, CuBiS₂

When using the mixtures and combinations described in the course of thetest outlined above, the following values are obtained:

    ______________________________________    Solid lubricant                dMy-v    dMy-p    PW-v   PW-p    ______________________________________    Mixture 2   -0,02    0,10     5,3    8,4    Mixture 3   0,0      0,10     7,5    15,7    Mixture 4   0,0      0,0      6,0    6,3    Mixture 5    0,05        0,01     5,2      6,1    Mixture 6   -0,08    -0,01    6,2    6,8    Products according to the invention:    Combination 2                -0,02    0,06     5,5    5,8    Combination 3                -0,04    0,06     6,7    13,5    Combination 4                -0,04    0,0      4,8    5,1    Combination 5                -0,12    -0,03    4,2    5,2    Combination 6                -0,15    -0,03    5,7    6,1    ______________________________________

The examples given show the improvements of the combinations accordingto the invention as compared to the binary mixtures, wear values as wellas for the stability of the coefficient of friction are influencedfavourably.

Of particular conspicuousness in the pure substances shown ascomparative examples is the enormous potential for stabilisation of leadsulphide, which in particular under speed load leads to a coefficient offriction increasing with temperature and thus to over-stabilisation.Pressure load also leads to over-stabilisation of the coefficient offriction, though to a lesser extent. The other binary sulphides listedabove show significant drops of the coefficient of friction and partlysignificantly higher values of wear. By mixing different sulphidesappropriately, the weaknesses of the individual sulphides may partly becompensated for, but the quality of lead sulphide cannot be achieved inthis way. Only by concertedly building-up the inventive ternary andquaternary phases in the mixture is it possible to generate combinationswhose quality equals the characteristics of heavy metal sulphides knownto be good.

I claim:
 1. A friction lining mixture containing, as a solid lubricantadditive, at least one compound of the formula

    M1.sub.l M2.sub.m M3.sub.n S.sub.x

wherein M1, M2 and M3 are different from each other and are selectedfrom the group consisting of Ti, V, Mn, Fe, Cu, Zn, Mo, W, Sb, Sn andBi, S is sulfur, l is from 1 to 5, m is from 1 to 5, n is from zero to5, and x is from 2 to
 8. 2. A friction lining mixture according to claim1 wherein the lubricant additive comprises a polyphase sulfide mixture.3. A friction lining mixture according to claim 2 wherein the lubricantadditive comprises a mixture of compounds of the formula M1_(l) M2_(m)M3_(n) S_(x).
 4. A friction lining formed from a friction lining mixtureaccording to claim 1, 2 or
 3. 5. A friction lining according to claim 4formed from a friction lining mixture which is resin-bound.
 6. Afriction lining according to claim 5 comprising a brake lining or clutchlining.